Children with brain tumors are generally followed by imaging studies, such as MRI. Difficulty arises in trying to distinguish tumor regrowth from treatment-related edema, necrosis or radiation injury. Proton nuclear magnetic resonance (NMRS) imaging is a noninvasive method of detecting and measuring cellular metabolites in vivo. NMRS complements MRI by giving chemical information in conjunction with spatial information obtained by MRI. This study is being conducted to determine NMRS imaging patterns before, during and after chemotherapy in pediatric patients with brain tumors in an effort to identify and characterize specific patterns of metabolites related to tumor regrowth, response to therapy, edema or necrosis.